The present invention relates generally to inkjet printing mechanisms, and more particularly to a bellows capping system for sealing an inkjet printhead during periods of printing inactivity.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.
During periods of printing inactivity, inkjet printheads are typically capped to prevent them from drying out, with the capping reducing evaporation of the ink components, as well as to protect the printhead from contamination due to environmental factors, such as dust, paper particles and the like. To form a good seal, the cap must conform to the printhead and supply enough force against the printhead to limit air transfer. Traditionally, capping has been accomplished using a compliant elastomer that is pressed against the printhead to create a complete seal.
Traditional inkjet capping solutions have used a vertical beam of elastomer that is pressed against the pen with considerable force, typically greater than 600 grams. Indeed, the forces on some pens may reach as much as 1200 grams or more due to variations in manufacturing tolerances, as well as whether the pen is properly seated against the carriage alignment datums, particularly in multi-pen systems. For instance, in a multi-pen system, one pen may be seated more deeply against the pen alignment datums than the remaining pens, leading to uneven capping forces where the more deeply seated pen receives a higher capping force than the pen which is not seated tightly against the datums. In extreme cases, very high capping forces may ultimately damage the delicate printhead orifice plate through which the ink ejecting nozzles are formed. In other cases having multiple printheads, the cumulative force experienced by one pen may actually exceed a printer""s capability to maintain pen alignment and other critical specifications, actually causing the pen to be unseated from the alignment datums. To alleviate these various ills, both pen designers and printer designers look to the service station cap designers to accommodate these manufacturing and installment variations while avoiding damage to the pens.